Anybody else noticed that?
Well here's what happened and I came across this discovery. I forgot to put equipment on Kyril, in the battle in Small Border Village, so he got defeated before crossing the line where the first chest can be found. Since I have collected all "S" items I didn't bother to reset the battle so I continued normally. During the battle those 3 characters never appeared, so I suppose you must keep Kyril alive before reaching that certain line. So yeah they are missable characters unfortunately, and you will miss the 2 Giant Owls as well.
Also there's a special dialogue as well I've noticed. I had Kika in my party in my previous playthrough, and I triggered some different dialogue between Kika, Frederica and Paula. Although surprisingly enough Frederica wasn't in my party. So I guess you will get it only if you have Kika (maybe you must upload SIV data as well, although I'm not 100% sure).